WELLFLEET - Over the past five weeks, 178 dolphins have stranded on Cape Cod. Most have been found dead, but the painstaking process of tending, hauling, and releasing the live ones is exacting a physical and emotional toll that grows greater every day. It is a toll made all the heavier because the reasons for the strandings remain a mystery.

Despite all the blood samples, the necropsies, and what will be a mountain of new data on the animals, known as common dolphins, researchers have no answers for the strandings, the largest involving a single species in the Northeast in at least two decades.

Ultrasounds are taken, bacterial and viral screenings are performed, blubber thickness is measured, and the sex and weight of each animal is meticulously logged.

Tissue samples are being analyzed for disease, biotoxins, and problems caused by humans, such as pollution and boat collisions. Six survivors have been tagged with transmitters to record their movement by satellite. So far, they have ventured as far as Maryland to mid-coast Maine.

“We’ve also been compiling some other environmental data, such as wind, currents, and sea-surface temperature,’’ said Mendy Garron, regional marine mammal stranding coordinator for the National Marine Fisheries Service.

But as far as answers?

“Gosh, it’s a puzzle. It’s really strange,’’ said Richard Connor, a biology professor at the University of Massachusetts Dartmouth who has studied bottleneck dolphins for 20 years in Australia.

“I think it’s one of those things we won’t be able to determine from one single event,’’ said Brian Sharp, stranding coordinator for the animal welfare group, as he studied reams of data in a Wellfleet coffee shop.

The strandings, which began Jan. 12, are nearly five times the average annual beachings of common dolphins on Cape Cod.

Of the 178 dolphins reported stranded through yesterday, primarily in an arc from Dennis to Wellfleet, 107 had died by the time rescuers reached them. Among the 71 found alive, 53 have been released into the ocean, mostly in the open waters off Provincetown.

Familiar theories from past strandings can be heard in speculation about this one: Cape Cod’s hooked shape makes exiting the bay difficult; rapidly falling tides can trap dolphins on the sand; and shallow creeks and inlets can interfere with sonar-like capabilities and leave dolphins confused and off course.

And because of their highly social nature, researchers said, one or two sick dolphins might lead many more astray.

But scientists are hampered by a lack of knowledge about this species, which roams in deep water from the mid-Atlantic coast to Canada. And, further deepening the mystery, most of the stranded dolphins have been healthy.

“It’s not like you can get permits to scoop up a lot of wild dolphins in the ocean,’’ said C.T. Harry, assistant stranding coordinator for the animal-welfare group.

To expand their understanding about the common dolphin, researchers are gathering fresh measurements, observations, and samples taken in two large trailers that act as emergency rooms for the animals.

“It’s about as close as you can get to a mobile dolphin clinic,’’ said Harry, 32, as he stood on a foam mat that can support an animal in distress. “We’ve got about everything you need but a coffee maker.’’

Once the rescuers receive a stranding report from a network of 300 volunteers, speed becomes critical.

Dolphins usually survive only several hours out of water, eventually succumbing to stress, injuries, organ failure, and the burden of carrying their own weight.

During this narrowing window, an intense choreography begins that combines triage, sample extraction, and back-straining transport to trailers and deeper water.

Connor, the UMass professor, said DNA samples can help determine if the stranded dolphins belong to one large group or subspecies whose specific biology might hold a clue to the phenomenon.

“If they just get the DNA and sex and size composition,’’ Connor said of the rescue team, the potential exists for an “incredible breakthrough’’ into the social structure of this species.

Animal-welfare activists concur that the data being retrieved almost daily will yield the most information ever collected on common dolphins. And that data, they argue, can shine an important light on the health of the planet.

“These animals make a living in a very dynamic environment. They’re a great sentinel,’’ said Harry, the assistant stranding coordinator. “This provides a huge data set for us.’’

The strandings have highlighted the affection many humans hold for dolphins. On Tuesday, Swede Pault of Wellfleet and Mike Giblin of Eastham knelt on a tidal flat as they tended to one of 10 dolphins that quivered from stress atop black, oozing mud near the Herring River.

Pault, 61, held a compress on the dolphin’s tail after blood had been drawn, while Giblin, 64, gently held its side.

“It’s hard work, but when you release them it’s wonderful,’’ said Giblin, a retired teacher. “There’s nothing better than this.’’

As the tide advanced, saltwater rose around the dolphins, already fitted with identification tags.

The decision had been made to refloat the animals here, instead of transporting them to Provincetown, because of the fast-moving tide and the difficulty of extracting them from mud.

But rising water itself did not guarantee success. With creeks, sandbars, and inlets, the passage to Cape Cod Bay was a meandering maze for which the dolphins would need guidance.

So, in a maneuver more suited to herding cattle, the animal-welfare group and the Wellfleet harbormaster used two boats to coax the dolphins toward freedom. Progress was slow, hampered as the dolphins played an error-filled game of follow-the-leader.

“You’re going the wrong way, buddy!’’ Moore said at one point, sloshing through thigh-deep water as she lunged for a dorsal fin.

By evening, all 10 dolphins were heading toward the bay. Although one other dolphin had died, this day had been a resounding success. At daybreak, the tiring and confounding work would begin again.